Lunar exospheric argon modeling

Highlights

• We have modeled the lunar argon (⁴⁰Ar) exosphere as constrained by Apollo 17 LACE measurements.

• The trapping in the PSRs is a sink for ⁴⁰Ar comparable to both photo-ionization and charge-exchange with solar protons.

• The density of ⁴⁰Ar measured by LACE appears to have originated from no less than four moonquakes.

• The area of the PSRs trapping ⁴⁰Ar is consistent with the presence of adsorbed water in such PSRs.

• ⁴⁰Ar densities appear to be in agreement with upper limits from LRO/LAMP and preliminary results from LADEE/NMS.

Abstract

Argon is one of the few known constituents of the lunar exosphere. The surface-based mass spectrometer Lunar Atmosphere Composition Experiment (LACE) deployed during the Apollo 17 mission first detected argon, and its study is among the subjects of the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and Lunar Atmospheric and Dust Environment Explorer (LADEE) mission investigations. We performed a detailed Monte Carlo simulation of neutral atomic argon that we use to better understand its transport and storage across the lunar surface. We took into account several loss processes: ionization by solar photons, charge-exchange with solar protons, and cold trapping as computed by recent LRO/Lunar Orbiter Laser Altimeter (LOLA) mapping of Permanently Shaded Regions (PSRs). Recycling of photo-ions and solar radiation acceleration are also considered. We report that (i) contrary to previous assumptions, charge exchange is a loss process as efficient as photo-ionization, (ii) the PSR cold-trapping flux is comparable to the ionization flux (photo-ionization and charge-exchange), and (iii) solar radiation pressure has negligible effect on the argon density, as expected. We determine that the release of 2.6 × 10²⁸ atoms on top of a pre-existing argon exosphere is required to explain the maximum amount of argon measured by LACE. The total number of atoms (1.0 × 10²⁹) corresponds to ∼6700 kg of argon, 30% of which (∼1900 kg) may be stored in the cold traps after 120 days in the absence of space weathering processes. The required population is consistent with the amount of argon that can be released during a High Frequency Teleseismic (HFT) Event, i.e. a big, rare and localized moonquake, although we show that LACE could not distinguish between a localized and a global event. The density of argon measured at the time of LACE appears to have originated from no less than four such episodic events. Finally, we show that the extent of the PSRs that trap argon, 0.007% of the total lunar surface, is consistent with the presence of adsorbed water in such PSRs.

Introduction

The lunar atmosphere was first detected by Apollo 12, 14 and 15 with the Cold Cathode Gauge Experiments (CCGE) deployed on the lunar surface. These CCGE measurements determined a density of 10⁷ cm⁻³ and 2 × 10⁵ cm⁻³ in daytime and nighttime, respectively (Johnson et al., 1972). CCGE showed a large day/night density ratio, opposite to what is expected for non-condensable gases. Therefore, it was clear that the dominant gases in lunar atmosphere were adsorbed at night and released on the dayside.

The Lunar Atmosphere Composition Experiment (LACE) was a mass spectrometer deployed on the lunar surface in December 1972 during the Apollo 17 mission as part of the Apollo Lunar Surface Experiments Package (ALSEP). LACE was the first and, until very recently (Benna et al., 2014a), the only instrument to convincingly detect argon in the lunar exosphere. The argon density at the Apollo 17 site was seen to vary cyclically and also to show an overall decrease in density during 9 lunations. Fig. 1 of Hodges and Hoffman (1974) is the only published measurement of lunar argon during all 9 lunations, although efforts to restore the ALSEP data stream are ongoing (Williams et al., 2013). It was soon clear that argon showed the typical behavior of a condensable gas given the cold nighttime surface temperature of the Moon. The density decreased soon after sunset because of the increasing adsorption, and increased before sunrise due to atoms coming from the already illuminated portion of the dayside surface. This pre-sunrise increase of argon occurred 50 km before the terminator (Hodges, 1973) because LACE was located in the floor of Taurus–Littrow valley and the mountains to the East delayed the illumination of the site by 8 h (Hoffman et al., 1973). There is also a smaller peak at sunset, resulting from a contribution of atoms migrating from the hot dayside. The fact that the peaks are not symmetric, as one might expect from the T^−5/2 dependence of the density in the horizontal transport assumption (Hodges and Johnson, 1968), is due to sequestration at nightside, which increases with decreasing temperature, and, to a lesser extent, at dayside microscale cold-traps (Henderson and Jakosky, 1997, Paige et al., 2010).

On the Moon, argon is released from the interior by radioactive decay of ⁴⁰K. Therefore, its production depends on the quantity of potassium present in the crust. With a concentration of K within the crust of 100 ppm (Taylor and Jakes, 1974), the argon production within the Moon was inferred to be 2.4 × 10²² atoms s⁻¹ (Hodges, 1975). From simulations it was initially determined that effusion of argon from the interior to the atmosphere was ∼2 × 10²¹ atoms s⁻¹, meaning that 8% of the argon production rate for the entire Moon escapes to the exosphere while the majority of argon atoms is retained within the crust.

This effusion rate was later revised to be 1.4 × 10²¹ atoms s⁻¹ (Hodges, 1977), corresponding to 3 ton/year. However, the mechanism for release for such a large amount of argon remained uncertain. Hodges (1977) initially excluded diffusion among the sources, on the basis that (a) it is too slow and (b) the returned lunar rock samples would show a paucity of ⁴⁰Ar instead of the excess reported by Heymann and Yaniv (1970). Hodges (1977) further concluded that argon must be released from small, warm regions at greater depth than the crust, in the molten asthenosphere, ∼1000 km in depth, where Latham et al. (1973) identified a highly attenuating zone for seismic shear waves. This depth has been recently revised to be 1250 km by Weber et al. (2011).

The deep source origin for argon was first questioned by Hodges (1981) and, 25 years later, by Killen (2002). Applying a sophisticated multipath diffusion code, Killen (2002) showed that diffusion from the crust (i.e., a source much closer to the surface, ∼25 km) could account for the effusive flux of argon into the lunar atmosphere. The proposed mechanism was the release of argon from opening of micropores and cracks, i.e. natural diffusion out of grains to pore spaces in the rocks with subsequent spilling into the exosphere after shallow moonquakes. A deep source was no longer necessary. We discuss moonquakes in more detail in Section 4.1 in light of our model’s assumed population being consistent with the amount of argon that can be released during a moonquake.

Fig. 1 in Hodges (1975) shows two diurnal profiles of the densities of argon measured by LACE just above the surface during lunations separated by 120 days, starting with the maximum argon density measured during the month of April 1973. We report it in Fig. 1.¹ The argon density is seen to decrease by a factor of ∼2 in 120 days, with minor short-term variations in the intervening lunations. Our modeling work aims to identify what caused this decrease. As the past works based on these measurements, we assume that there was no degradation in the instrument, and that all calibration errors were taken into account (for a detailed description of the calibration of LACE, see Hoffman et al., 1973).

Loss of argon through ionization by solar UV photons is far more efficient than gravitational escape, and has previously been determined to be the primary loss mechanism (Hodges, 1977, Killen, 2002). Once argon atoms are ionized, the photo-ions are instantly entrained by the convective electric field E_sw = −v_sw × B_IMF (v_sw is the velocity of the solar wind, and B_IMF is the interplanetary magnetic field). Roughly half of these photo-ions are thought to impact the surface and to be neutralized because of their large gyroradius (Manka and Michel, 1970) combined with the low scale height of neutral Ar (in fact, the pickup ion velocity distribution is dependent upon the velocity of the ion and the gyroradius/scale height ratio, which in turn varies as the mass squared (Hartle and Killen, 2006, Hartle et al., 2011). This process is termed “recycling” since these particles may again be released from the surface as neutrals. The short-term variations from lunation to lunation were first (Hodges, 1977) attributed to High-Frequency Teleseismic (HFT) events measured by other Apollo stations (Nakamura et al., 1974) but Hodges (1980) proposed that seasonal (i.e., 1–10 years) storage of argon in Permanently Shaded Regions (PSR) could explain, at least partially, the time variations measured by LACE. The trapped argon would then be occasionally released from PSRs following shallow moonquakes, meteor impact, or seasonal warming of polar caps (Hodges, 1982). The argon PSR cold-trapping hypothesis followed the analogy for water retention in PSRs, which was proposed well before the beginning of the Apollo program itself (Watson et al., 1961, Arnold, 1979). Hodges (1980) demonstrated that argon retention on water contaminated rocks for a year or more is mainly in doubly shielded regions of large, flat-floored craters located at latitudes greater than 75°. The area affected by this argon retention is 0.5% of the area of the lunar surface at latitude greater than 75°, or about 0.05% of the total lunar surface. The reason for this double-shielded argument is that the heat reradiated by nearby orographic features (such as rims exposed to sunlight) would liberate argon from the surface. More recent observations with LRO/LOLA and modeling (Mazarico et al., 2011) suggest PSRs cover an area of ∼13,000 and ∼16,000 km² at the North and South Poles, respectively, or ∼2% and 2.5% of the area of the cap within 15° from each pole. The newly defined PSRs areas translate into 0.03% and 0.04% (for the N and S Poles respectively) of the total lunar surface, which we refer to here. In comparison, Watson et al. (1961) previously estimated an area 10 times too high, i.e. 0.5% of the total lunar surface, when analyzing photographs taken from the Earth.

The LAMP (Lyman Alpha Mapping Project) UV spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) is studying the far-UV reflectivity of the PSRs using illumination by starlight and interplanetary Lyman-α skyglow to investigate their volatile content and distribution (Gladstone et al., 2012). Argon is thought to be among the stored volatiles and its temporary storage could in theory account for the variation in density observed by LACE (Hodges, 1975). Therefore, a transport and storage model is required in order to study the behavior of argon, whose lateral migration likely resembles other important volatiles, like water, which have been modeled in recent years (e.g. Hodges, 2002, Crider and Vondrak, 2002, Schorghofer and Taylor, 2007, Hurley, 2010, Farrell et al., 2013).

Further detections of lunar argon from space-based experiments have, until recently, proven elusive. The mass spectrometers flown during Apollo 15 and 16 had no success in determining upper limits for specific neutrals (Hodges et al., 1972, Stern, 1999). Detection of argon from the UV spectrograph onboard the ORFEUS – SPAS II satellite was claimed by Flynn (1998), but was subsequently rebutted by Parker et al. (1998). Recently, LRO/LAMP has placed only an upper limit of 2.4 × 10⁴ cm⁻³ for argon near the poles (Cook et al., 2013). The LAMP argon limits are 5% and 38% lower than LACE measurements two-hours before and after the dawn and dusk terminators, respectively, in the comparable local time regions measured by LAMP in LRO’s polar orbit. Sridharan et al. (2013) published a study on the ⁴⁰Ar/³⁶Ar ratio obtained by Chandrayaan-1, but they do not report the ⁴⁰Ar density. Finally, preliminary results from the LADEE spacecraft confirmed the detection of argon near the sunrise terminator (Benna et al., 2014a) for the first time after four decades.

We address here the expected density of argon to aid ongoing interpretation of LRO/LAMP and LADEE measurements. Consideration of radiative transfer effects is needed to provide expected argon brightness and is left for future LRO/LAMP argon observation papers.

In Section 2 we describe the exosphere model, which is built upon the temporal Monte Carlo method described in Chaufray et al. (2009). Section 2.2 describes the incorporation of a LRO/Diviner surface temperature map while Sections 2.3 The residence time, 2.4 Model – data comparison explain our ad hoc method for representing the residence time of argon atoms as a function of this temperature using the LACE determined diurnal behavior itself. Argon loss and source processes are described in Sections 2.5 Argon loss processes, 2.6 Argon source processes respectively. In Section 3 we organize our model runs with increasing complexity, starting with loss by solar photo-ionization and solar wind proton charge exchange alone and then including additional loss by PSR sequestration. Our discussion in Section 4 is organized with a continued discussion of the initial production of argon by moonquakes, the implications of argon trapped in PSRs, and the study of a localized release of argon. We list four main findings in Section 5 together with recommendations for future LRO/LAMP and LADEE measurement comparisons.